Slip casting is a kind of molding technique of powders applied, in particular, to the field of ceramics and employed from old in potteries for sanitation fixtures, chinas, porcelains and the like. In recent years, this technique is applied also to the field of fine ceramics using alumina (Al.sub.2 O.sub.3), silicon nitride (Si.sub.3 N.sub.4) and the like and is expected to be a molding technique of parts for industrial use.
Gypsum molds known from the oldest time are conventional as a mold for slip casting. The reason therefor include excellent water-absorptivity, mold-releasability and dimensional accuracy as well as the shapeability of large and complicated forms at low cost.
On the other side, however, gypsum molds are poor in the water resistance, compressive strength and abrasion resistance and have a defect that the Ca.sup.2+ ions in the gypsum are dissolved out to enter the ceramic body. In addition, gypsum molds have a problem that the porosity and pore diameter distribution differ from mold to mold to cause variation in the water absorptivity so that definite molding conditions cannot be undertaken. When the mold is used in molding under pressurization in order to increase the productivity, furthermore, the strength is insufficient and a defect is caused that a large number of the molds must be used in parallel.
Standing on such a background, a variety of molds of other materials are under development with an object to overcome the defective points of gypsum molds. Major materials include resins, composite materials of a resin and a ceramic, composite materials of a metal and a ceramic, metals and the like. (Yoichi Motoki, "Manufacturing Process of Ceramics" published Oct. 10, 1978 by Gihodo Publishing; official publication of Japanese Patent Kokai 60-66405; and official publication of Japanese Patent Kokai 49-53216).
These molds, however, have the following defective points. (1) The pore diameter is larger than 3 .mu.m so that clogging takes place on the mold when fine particles are subjected to slip casting.
(2) The mold-releasability between the mold and the ceramic body is poor so that compulsory mold-takeoff must be undertaken by passing a gas and other means sometimes to cause damage on the molded body. When polyvalent cations highly ion-exchangeable, such as Ca.sup.2+ ions, are incorporated into the surface of the mold in order to improve the mold-releasability, they enter the ceramic body.
(3) Fabrication of the mold is not an easy matter and the working cost is high to impart full accuracy to the surface of the product or to finish a product of a complicated configuration having an extremely thin-walled portion or an extremely narrow portion with a good dimensional accuracy.